?Multigenerational epigenetic programming induced by paternal obesity and prediabetes? ABSTRACT Obesity and its related co-morbidities have reached epidemic proportions in the United States and other developed countries, posing an unprecedented challenge to health services. We find that a single generation of paternal obesity and prediabetes programs male offspring with a latent metabolic defect that is exposed by overnutrition. The latent phenotype is transmitted paternally for two generations without further exposure to paternal obesity. Paternal transmission is associated with changes in the sperm small RNAs that are predicted to regulate transcriptional processes. This non-genetic transmission of phenotypes across generations may be a significant contributor to the risk of obesity, and may require intervention and prevention measures across multiple generations. Understanding the scope and the mechanism of this heritable epigenetic programming phenomenon will be critical in developing new strategies to manage or prevent the effects of paternal obesity. This study has the potential to identify biomarkers that could be used to characterize the syndrome in humans. We propose to determine (1) if multiple generations of exposure to paternal obesity and prediabetes amplify the metabolic phenotype in subsequent generations, and/or increase its heritability; (2) the mechanism of paternal inheritance of the metabolic phenotype, through epigenetic changes in the sperm of affected sires; (3) the transcriptional changes that occur in the early embryo in response to epigenetic changes in the sperm. The concept that environmental exposures can induce heritable epigenetic states has received much recent attention, but the subject is very poorly understood and documented. Our experimental system permits a direct test of the epigenetic inheritance model: genetic variants can be ruled out as a factor in transmission because we study isogenic mice, transmission through the paternal line rules out in utero metabolic exposure as a cause, and the high penetrance of the metabolic phenotype makes it amenable to mechanistic studies. The unexpected multigenerational effect of paternal obesity/prediabetes on the metabolism of genetically identical offspring challenges established views on the causes of obesity. This study will provide evidence that there is an inborn but non-genetic component to the risk of obesity, insights into the mechanisms by which that risk is created and transmitted, and a system amenable to further study of the phenomenon.